When a captain releases their Shikai, they say a phrase that is unique to their Zanpakuto only, followed by the name of their Zanpakuto. For example, 10th Division Captain Toshiro Hitsugaya says "Sit upon the frozen skies, Hyorinmaru!" If it were the Head Captain Yamamoto, he would say "Burn all of creation to ashes, Ryujin Jakka!" This much goes the same way for every shinigami with a shikai, regardless of rank. For Bankai, you need only say "Bankai" to activate it, and then the user will follow by stating the complete name of their Bankai. For example: Byakuya Kuchiki would say "Bankai, Senbonzakura Kageyoshi", while Ichigo would say "Bankai, Tensa Zangetsu"You can find out by visiting their page on Bleach Wiki.